The cell cycle-dependent changes in the sensitivity of sea urchin eggs to the lethal effects of ionizing radiation and to radiation-induced mitotic delay will be investigated. The mitotic cycles of the male and female chromosomes will be separated by stimulating DNA synthesis in the maternal genome before fertilization. This technique is expected to reveal information regarding the relationships, if any between radiation-induced death and radiation-induced mitotic delay and the dependence of these two expressions of radiation damage on respiration, the synthesis of DNA and protein, the duplication and separation of centrioles, and chromosome condensation.